SendGrid 421/450 deferred: the receiving server is throttling you

TL;DR

Check the deferral rate per domain in your event data. Follow the warmup schedule instead of forcing volume.

Steps

1. Symptom: deferred events on the Event Webhook, or SMTP replies 421 / 450.
2. Do not resend manually. Deferred messages are retried automatically for up to 72 hours.
3. Reduce volume to the deferring domain. More mail to a throttling server deepens the deferral.
4. Check the deferral rate per domain in your event data. One hot domain means a throttling policy; all domains means your IP or content reputation.
5. If you are on a new dedicated IP, this is expected during warmup: ISPs throttle unknown senders. Follow the warmup schedule instead of forcing volume.
6. Persistent 421s with good content and a warmed IP point at engagement: low opens and high complaints teach ISPs to throttle you. Fix the list and the content, not the retry logic.
7. 4xx means try later, 5xx means stop. Never treat a 421 like a bounce.

When to use

You are seeing this: Symptom: deferred events on the Event Webhook, or SMTP replies 421 / 450. Use this skill when you run into "SendGrid 421/450 deferred: the receiving server is throttling you".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

The official SMTP errors doc: 421 'Message temporarily deferred' and 450 'too frequent connects' are recipient-server throttling; Twilio retries deferred messages for up to 72 hours. Sending more to a deferring domain makes it worse. Sources: http://sendgrid.com/docs/Glossary/emailerrormessages.html